1. Field of the Invention
The present invention relates to an isolator applied to an antenna combiner.
2. Description of the Related Art
A conventional isolator will now be described with reference to the accompanying drawings. FIG. 8 is an exploded perspective view of a conventional isolator. FIG. 9 is a plan view of main parts of the conventional isolator. FIG. 10 is a development view of central conductors related to the conventional isolator.
The structure of the conventional isolator will now be described with reference to FIGS. 8 to 10. The conventional isolator includes a boxlike first yoke 51, a disc-like magnet 52 arranged in the first yoke 51, a flat plate-shaped ferrite member 53 arranged under the magnet 52, first, second, and third central conductors 54, 55, and 56 made of a metal plate, which are mounted to the ferrite member 53 at intervals of 120° and parts of which cross each other, a boxlike resin case 57 that holds the ferrite member 53, and a U-shaped second yoke 58 arranged under the resin case 57.
The ferrite member 53 is of a rectangle having two long sides 53a opposite to each other and two short sides 53b opposite to each other.
As illustrated in FIG. 10, the first, second, and third central conductors 54, 55, and 56 are formed by punching a metal plate and are formed to extend outward from a square earth 70 provided in the center.
The first, second, and third central conductors 54, 55, and 56 are divided into two by slits 54a, 55a, and 56a provided in their longitudinal directions, respectively. The first, second, and third central conductors 54, 55, and 56 also have conductors 54b, 55b, and 56b, each consisting of two streaks of conductors having the same width and parallel to each other, and first, second, and third ports 54c, 55c, and 56c provided at the ends of the conductors 54b, 55b, and 56b. 
Further, the first, second, and third central conductors 54, 55, and 56 are arranged such that the earth 70 is first arranged on the bottom face of the ferrite member 53, and the first, second, and third central conductors 54, 55, and 56 are then bent along the side face and top face of the ferrite member 53.
Although not illustrated herein, the first, second, and third central conductors 54, 55, and 56 located on the top face of the ferrite member 53 are arranged in a vertical direction in a state of being insulated from each other by dielectric bodies.
When the first, second, and third central conductors 54, 55, and 56 are mounted to the ferrite member 53, the first and second central conductors 54 and 55 are located on the short sides 53b and are arranged to cross the long surface of the ferrite member 53. The third central conductor 56 is located on the long side 53a and is arranged so as to transverse the short surface of the ferrite member 53 parallel to the short side 53b. 
Further, the resin case 57 is provided with a bottom wall 57b having a hole 57a. Input and output terminals 59 and 60 and an earth terminal 61 are buried in the bottom wall 57b in a state where parts thereof are exposed to the outsides of the bottom wall 57b and the resin case 57.
The ferrite member 53 to which the first, second, and third central conductors 54, 55, and 56 are attached is arranged in the hole 57a so that the earth 70 at one end of each of the first, second, and third central conductors 54, 55, and 56 is connected to the second yoke 58.
Chip-type capacitors 62, 63, and 64 and a chip-type resistor 65 are arranged around the hole 57a so that electrodes on the bottom faces of the capacitors 62, 63, and 64 and an electrode 65a at one end of the resistor 65 are connected to the earth terminal 61.
The ports 54c and 55c of the first and second central conductors 54 and 55 are soldered to electrodes on the top faces of the capacitors 62 and 63. Further, the port 56c of the third central conductor 56 is connected to an electrode on the top face of the capacitor 64 and the top face of an electrode 65b at the other end of the resistor 65 by soldering.
The first and second yokes 51 and 58 are combined with each other in a state where the magnet 52, the ferrite member 53, and the resin case 57 are sandwiched between the first yoke 51 and the second yoke 58 so that a magnetic closed circuit is formed by the first and second yokes 51 and 58. Therefore, an isolator is formed (For example, refer to the Patent Document 1).
However, since the third central conductor 56 of a conventional isolator is provided on the long side 53a and is arranged so as to transverse the short surface of the ferrite member 53 parallel to the short sides 53b, the third central conductor 56 has a small inductance component. Therefore, it is necessary to increase the size of the capacitor 64 and to thus increase the size of the isolator.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2001-94311
In a conventional isolator, since the third central conductor 56 is provided on the long side 53a and is arranged so as to transverse the short surface of the ferrite member 53 parallel to the short sides 53b, the length of the third central conductor 56 is reduced. Therefore, the third central conductor 56 has a small inductance component. As a result, it is necessary to increase the size of the capacitor 64 and to thus increase the size of the isolator.